I've done the walk and the boat but didn't know there was a bus.
I'd say for the FASTEST, probably the bus.
If you are tired, then the boat is good. It takes a long time though. It's slow boat and it will stop at Swan/Dolphin, B/C, Boardwalk, and then finally Epcot. It takes at least 20 minutes or more.
If you are not tired, then the walk is a bit quicker then the boat. My DH always beats us if we take the boat and he walks. The walk is approx 15 minutes if you are a good walker; 20 minutes of you are tired at all.